There has been a conventionally known a driving wheel slip control device in which, in order to prevent an excessive slip of a driving wheel, the state of slip is detected in response to the velocity of a driving wheel and the like. When a slip has occurred, for example, fuel supplied to an engine is stopped in order to suspend the combustion of the engine. Japanese Patent Laid-Open Publication 47-1512 discloses a slip control device in which suspension of combustion of an engine is prohibited when the engine is rotated at a low speed or when a vehicle runs at a low speed in order to prevent an engine stall.
However, in conventional slip control devices, a region in which the combustion of the engine is prohibited from stopping (this region is simply called "a prohibition region" hereinafter) is determined on the basis of either the number of revolutions of an engine or the velocity of the vehicle. Therefore, a defect occurs because (a) a suspension system resonates which decreases the driving ability of the vehicle and (b) slip control cannot be carried out over a wide region of revolutions of an engine while preventing an engine stall.
That is, in case the prohibition region is determined on the basis of only the number of revolutions of an engine, and a predetermined number of revolutions of an engine, as a threshold value, is determined at a minimum value to prevent engine stall, the cycle of combustion and the stopping of combustion of the engine will coincide with a specific frequency of the suspension system, when the vehicle is started or during low velocity of the vehicle. This generates a resonance of the suspension system which decreases the driving ability of th vehicle. Moreover, in case the predetermined number of revolutions of an engine is determined at a value higher than the minimum value in order to prevent such a resonance, the prohibition region is widened to include a region of middle velocity of the vehicle in a high gear position of a transmission gear. This will reduce the region where slip control is to be primarily performed which corresponds to an expansion of the prohibition region.
In contrast, in case the prohibition region is determined only on the basis of the velocity of the vehicle, the region of low engine revolution is excluded from the prohibition region in a high gear position of the transmission gear which makes it impossible to prevent an engine stall.